When a network needs to transfer volumes of information over long distances, RS-485 is often the communications standard of choice. The network nodes may be personal computers, microcontrollers, or any device capable of asynchronous serial communication. Compared to Ethernet and other network interfaces, the hardware and protocol requirements of RS-485 are simpler and cheaper.
1. What is RS-485 communication?
In 1983, the Electronic Industry Association (EIA) approved a new balanced transmission standard called RS-485. Has been widely accepted and used in industry, medical, and civil. The RS485 standard can be considered as a development of RS232 in serial data transmission. These RS232 / RS485 converters allow users to communicate with any device that uses the RS232 serial link via RS485. RS485 link is formed for long distance data acquisition and control for applications. The outstanding feature of RS485 is that it can support a network of up to 32 base stations on the same line, the baud rate can be up to 115,200 for a distance of 4000feet (1200m).
With the balanced transmission pattern and the wires are twisted together, when the noise occurs in one wire, it also happens in the other wire, which means that the two wires have the same noise. This makes the voltage difference between the two wires do not change significantly, so the receiver still receives the correct signal thanks to the special feature of the receiver that eliminates noise. The RS485 link is widely used in industry, where the noise environment is quite high and confidence in system stability is important. Besides, the ability to communicate over long distances at high speed is also very interested, especially in places where many communication stations are spread out over a wide area.
2. Characteristics of RS-485 communication
2.1 Balance transmission
Balanced transmission system consists of two signal wires A, B but no ground wire. The reason is called balance because the signal on one line is opposite to the signal on the other. This means that one wire is playing high, the other is playing low and vice versa.
2.2 Signal level
With two wires A and B being balanced, the TTL high-level signal is specified when the voltage of wire A is at least 200mV from wire B, the TTL low-level signal is specified when the pressure of wire A is smaller than wire B minimum is also 200mV. If the VAB voltage is within -200mV <VAB <200mV then the signal is now considered to fall into the indefinite region. The voltage of each signal wire compared to the signal ground on the receiver side must be within – 12V to + 12V.
2.3 Twisted-pair wire
+ As its name suggests, twisted-pair wire is simply a pair of wires of equal length and twisted together. Using twisted pair will minimize noise, especially when traveling over long distances and at high speeds.
+ Twisted pair wire characteristic impedance:
Depending on the shape and insulation of the wire, it will have a Characteristic impedence (Zo), which is usually indicated by the manufacturer. The recommended characteristic impedance for a line is between 100 and 120 ohm, but this is not always the case.
2.4 Common type voltage
The transmission signal consists of two wires without a mass wire, so they need to be referenced to a common point, the common point at this time can be the mass or any given voltage level. The common-mode voltage (VCM) is mathematically stated as the average of the two signal voltages referenced to mass or to a common point.
The signal on the two wires when referenced to the common ground is then it needs to be scrutinized. At this time the receiver will determine the signal by reference that signal to the ground of the receiver, if the ground between the receiver and the transmitter has a voltage difference that exceeds the allowed threshold, the received signal will be wrong. or damage the device. This shows that the RS485 network consists of two wires but up to three voltage levels are considered. Because ground is an imperfect electrical conductor, it has a definite resistance, which causes a potential difference from point to point, especially in areas with lots of thunder, large current consuming machines, These converters are installed and grounded.
The RS485 standard allows a difference of earth potential up to 7V, which is greater than 7V. So the ground is an unreliable reference point and a better way for signaling now is to add a third wire, which will be grounded mass at the source to use as a reference voltage.
2.6 Terminal resistors
Terminal resistors are simply resistors located at the two ends of the line. The ideal terminal resistance value is equal to the characteristic impedance value of the helix, usually around 100-120Ohm.